Dicamba is a selective herbicide currently used for treating e.g. corn, wheat or grassland. It kills broadleaf weeds before and after they sprout. The trivial name dicamba refers to the compound 3,6-dichloro-2-methoxybenzoic acid. It is expected that the global demand for dicamba will increase significantly.
Dicamba is typically produced on an industrial scale from 2,5-dichlorophenol using carboxylation under Kolbe-Schmitt conditions, methylation and subsequently saponification/acidification. 2,5-dichorophenol in turn can be obtained from 1,4-dichlorobenzene or 1,2,4-trichlorobenzene. The synthetic route via 1,2,4-trichlorobenzene suffers from limited availability of this compound and from the formation of several by-products which are formed in the synthesis of 2,5-dichlorophenol. The synthetic route via 1,4-dichlorobenzene involves nitration, reduction to 2,5-dichloroaniline, diazotation, and subsequent conversion of the diazonium salt to 2,5-dichlorophenol. However, due to the reactivity of the diazonium salt several by-products are also formed in the latter step so that the yield thereof is undesirably low. In addition, on an industrial scale major amounts of waste material are generated in this step.
Prior art processes for obtaining 2,5-dihalogen phenols, such as 2,5-dichlorophenol, from 2,5-dihalogen aniline are typically carried out batch-wise by heating the diazonium salt in an aqueous acid. Illustrative reaction conditions are e.g. described in DE 25 09 407. In conventional processes, the diazonium salt is typically not heated to temperatures higher than 140° C. The obtained yields are about 70%
In view of the above, there is a need in the art for an improved process for obtaining 2,5-dihalogen substituted phenols, such as 2,5-dichlorophenol. In particular, there is a special need in the art for processes providing 2,5-dihalogen substituted phenols in better yields. Furthermore, there is a need for such processes providing increased yield that advantageously can be carried out on an industrial scale. Moreover, such processes should preferably allow for the reduction of waste material and/or for recyclability of materials applied.
The object of the present invention is to meet the above needs. In particular, one object of the present invention is to provide an improved process for obtaining 2,5-dihalogen substituted phenols such as 2,5-dichlorophenol. The process according to the present invention can be carried out on an industrial scale. Furthermore, in a preferred embodiment, the process according to the present invention produces less waste material and allows for the recycling of at least part of the chemicals or materials involved in the process. In an especially preferred embodiment, the object of the present invention is the provision of a process for providing 2,5-dichlorophenol and finally dicamba on an industrial scale in good yields.